Although mitotic chromosome segregation has been well characterized cytologically, the molecular mechanisms that underlie this complex process remain obscure. Segregation errors during cell division have been determined to be the cause of various disorders including Down's syndrome and spontaneous fetal abortion. In addition, nondisjunction may play an important role in the promotion of neoplasia. The long-term objective of the proposed project is to gain a molecular understanding of the structures and events responsible for chromosome segregation in the yeast S. cerevisiae. We expect that a mitotic spindle requires the function of many, perhaps hundreds, of proteins to operate properly, yet at this time, only a handful of these proteins have been identified. We propose a genetic methodology that will efficiently identify a large number of the genes that encode yeast spindle proteins. Preliminary results indicate that this approach will be rewarding; many new genes, whose products are required for proper chromosome segregation, have been identified. Our specific experimental aims are: 1) the identification of mutants that show a reduced fidelity of mitotic chromosome transmission. We expect to find that many of these mutants are defective in the chromosome segregating machinery. The genes defined by these mutants will be molecularly cloned and characterized. 2) the phenotypic characterization of chromosome segregation mutants. To determine the role performed by the wild-type gene- products in chromosome segregation, the in vivo consequences of loss of gene-product activity will be examined by functional and morphological assays. In addition, mutants that block specific mitotic steps will be examined by order-of-function tests. 3) the identification, by three genetic methodologies, of genes whose products interact with the products of previously identified chromosome segregation genes. 4) determination of the intracellular locations of chromosome segregation proteins by cytological and biochemical methods.